Learn Hardcore Finnish: Tämän päivän sääennuste sanoo, että aamu on pilvinen mutta iltapäivä tuulinen. - Today’s weather forecast says that the morning is cloudy but the afternoon is windy.

we’re talking about the forecast belonging to this day, so the genitive is used, just like English uses ’s in today’s forecast. Finnish doesn’t usually say tämä päivä sääennuste; it needs that genitive structure.

What case is päivän, and why is it used here?

Päivän is the genitive singular of päivä (day).

The pattern is:

nominative: päivä – day
genitive: päivän – of (the) day

The genitive is used for:

Possession / belonging
- Tämän päivän sääennuste – this day’s weather forecast / today’s weather forecast
Noun–noun chains (like English N1 of N2 or N2’s N1):
- kaupungin kartta – map of the city / city map
- Suomen pääkaupunki – the capital of Finland

So here päivän marks that the sääennuste (forecast) is tied to this day.

Why is sääennuste used with sanoo (“says”)? Can a forecast really “say” something in Finnish?

Yes. In Finnish, it’s very natural for inanimate things like forecasts, books, signs, etc. to “say” something using sanoa.

Sääennuste sanoo, että… – The weather forecast says that…
Kyltti sanoo, että… – The sign says that…
Artikkeli sanoo, että… – The article says that…

You could also use other verbs:

sääennuste kertoo, että… – the forecast tells that…
sääennuste ennustaa, että… – the forecast predicts that…

But sääennuste sanoo is perfectly idiomatic and common.

What does että do in this sentence, and why is there a comma before it?

Että is a subordinating conjunction meaning roughly “that” (as in “X says that Y…”).

Structure:

Tämän päivän sääennuste sanoo, että aamu on pilvinen…
– Today’s weather forecast says that the morning will be cloudy…

The part after että (että aamu on pilvinen mutta iltapäivä tuulinen) is a subordinate clause (a content clause) giving the content of what the forecast “says”.

Finnish almost always puts a comma before että when it starts such a subordinate clause, even in cases where English would not always put a comma.

Why is there only one on in aamu on pilvinen mutta iltapäivä tuulinen? Is on omitted on purpose?

Yes, the second on is simply left out because it is understood. This is normal in Finnish.

Full version:
… että aamu on pilvinen mutta iltapäivä on tuulinen.
Common, natural version:
… että aamu on pilvinen mutta iltapäivä tuulinen.

When you have the same verb repeated in a parallel structure, Finnish often drops the second (and third, etc.) occurrence if it is clear from context:

Aamu on pilvinen ja iltapäivä (on) tuulinen.
Hän on väsynyt ja sinä (olet) iloinen.

Both forms are grammatically correct; the shorter one is just more economical.

Why pilvinen instead of pilvistä? I’ve seen both when talking about cloudy weather.

Both exist, but they are used in different structures.

Pilvinen – adjective (cloudy)
Used as a predicate adjective with olla:
- Aamu on pilvinen. – The morning is cloudy.
- Iltapäivä on tuulinen. – The afternoon is windy.
Here aamu and pilvinen agree in case and number (both nominative singular).
Pilvistä – partitive form of the noun pilvi (cloud)
Used in impersonal weather expressions:
- On pilvistä. – It is cloudy. (literally “there is cloudiness”)
- Aamulla on pilvistä. – In the morning it is cloudy.

So:

Aamu on pilvinen. – describes the morning as a cloudy period.
Aamulla on pilvistä. – says that during the morning the weather is cloudy.

In your sentence, we’re describing aamu and iltapäivä as entities with properties, so pilvinen and tuulinen are the right forms.

What’s the difference between aamu, aamupäivä, and iltapäivä?

These are different parts of the day:

aamu – morning, roughly from when people wake up until maybe 9–10 a.m.
aamupäivä – literally “morning-day”, often used for late morning, say ~9–12.
iltapäivä – literally “evening-day”, but actually afternoon, roughly 12–17/18.

In everyday use the boundaries are fuzzy, but:

If you say aamu on pilvinen, you’re talking about the earlier part of the day.
If you want to be more precise about late morning vs afternoon, you might say, for example:
- Aamupäivä on pilvinen mutta iltapäivä tuulinen.

What does sääennuste literally mean, and is it a compound word?

Yes, sääennuste is a compound:

sää – weather
ennuste – forecast, prediction

So sääennuste literally is “weather forecast”.

You might also see related words:

ennustaa – to predict, to forecast
ennustus – a prediction, often with a more “prophecy / guess” feel
sääennustus exists but is much less common and can sound slightly less standard or more colloquial; sääennuste is the normal word for a weather forecast.

How are pilvinen and tuulinen formed, and do they belong to some pattern?

Yes. They are adjectives formed with the suffix -inen, which often means “having X / characterized by X”.

pilvi (cloud) → pilvinen (cloudy, full of clouds)
tuuli (wind) → tuulinen (windy, full of wind)

Other common examples:

lumi (snow) → luminen (snowy)
onni (luck, happiness) → onninen is not used, but onnellinen (happy) uses a related pattern
suola (salt) → suolainen (salty)

So -inen is a very common adjective-forming suffix in Finnish, which is why you see it a lot in weather words like pilvinen, tuulinen, luminen, aurinkoinen (sunny).

Could I say Tänään sääennuste sanoo, että… instead of Tämän päivän sääennuste sanoo, että…?

You can say Tänään sääennuste sanoo, että…, but it changes the nuance:

Tämän päivän sääennuste sanoo, että…
– Today’s weather forecast says that…
Focus: the forecast for this day.
Tänään sääennuste sanoo, että…
– Today, the weather forecast says that…
Focus: what the forecast happens to say today (maybe compared to what it said yesterday).

They are both grammatically correct. If you mean the forecast for today’s weather, Tämän päivän sääennuste is the most natural and precise phrase.

Can I change the order inside the että-clause, like putting iltapäivä first?

Yes, Finnish word order is quite flexible, especially with parallel elements like this. All of these are possible:

… että aamu on pilvinen mutta iltapäivä tuulinen.
… että iltapäivä on tuulinen mutta aamu pilvinen.
… että aamu on pilvinen ja iltapäivä on tuulinen.

The main effect of changing order is focus or emphasis, not basic meaning. For example:

Starting with iltapäivä might highlight the afternoon more (e.g. if that’s the surprising part).
Repeating on can make the sentence a bit slower and clearer, but not more correct.

So you can move these parts around quite freely as long as the sentence remains clear.